A ring resonator is a waveguide formed in a closed loop. Light can be coupled from a second waveguide placed close to the ring resonator. At resonant wavelengths of the ring resonator, optical power from the second waveguide develops as a traveling wave in the ring resonator. However, light propagating at non-resonant wavelengths in the second waveguide continues to propagate with no coupling effect to the ring resonator. The resonant wavelength of the ring resonator can be tuned by changing the effective refractive index of the waveguide.